The diffusion coefficients of polystyrene (PS) in decahydronaphthalene (DHN) and in solutions of carbon dioxide (CO2) and DHN were measured for dilute PS solutions over a range of temperatures and CO2-DHN ratios using high pressure dynamic light scattering. Infinite dilution diffusion coefficients (DO) of PS and dynamic second virial coefficients (k(D)) were determined for essentially monodisperse 308 kDa PS. At a system pressure of 20.7 MPa, PS diffusion coefficients increased by a factor of 2.5, and the activation energy of diffusion decreased by approximately 16% when DHN was "expanded" with 44 mol% CO2. However, the hydrodynamic radius of PS at a given temperature was not particularly sensitive to the CO2 concentration. Solvent quality, as measured by kD, decreased at higher CO2 concentrations. The addition of CO2 to polymer solutions may offer a way to "tune" the properties of the solution to facilitate the heterogeneous catalytic hydrogenation of polymers. (C) 2009 Elsevier Ltd. All rights reserved.